MacOSVersion: enable strong typing, expand tests

[samford]

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This upgrades MacOSVersion to typed: strong and adds tests to expand coverage to 100% for lines and branches. The changes outside of MacOSVersion are fixes for related type issues that were surfaced after expanding type signatures and upgrading the type sigil.

This was originally part of some larger typing work (in Cask) but it may benefit #20096, so I figured I would extract this into its own PR to get it out sooner.